How Domestic Varieties Originate 239 



clusivqly as a glass-house flower. So the carnation types 

 of Europe and America have been widely unlike. 



Sowing the seeds of hardy annual plants in autumn 

 often stimulates a tendency to produce thickened roots. 

 The plant, rinding itself unable to perfect seeds, stores its 

 reserve in the root, and it therefore tends to become 

 biennial. In this manner, with the aid of selection and 

 the variation of the soil, Carriere was able to produce 

 good radishes from the wild slender-rooted charlock 

 (Raphanus Raphanistrum) . 



Lessened vigor, so long as the plant continues to be 

 healthy, nearly always results in a comparative increase of 

 fruits or reproductive organs. It is an old horticultural 

 maxim that checking growth induces fruitfulness. It is 

 largely in consequence of this fact that plants bear heaviest 

 when they attain approximate maturity. Trees are 

 often thrown into bearing by girdling, heavy pruning, 

 the attacks of borers, and various accidental injuries. 

 The gardener knows that if he keeps his plants in vigorous 

 growth by constantly putting them into larger pots, he 

 will get little, or at least very late, bloom. The plant- 

 breeder, therefore, may be able to induce the desired 

 initial variation by attention to this principle. (See dis- 

 cussion of variation in relation to food supply.) Arthur 

 has recently put the principle into this formula: "A 

 decrease in nutrition during the period of growth of an 

 organism favors the development of the reproductive 

 parts at the expense of the vegetative parts." 



A most important means of inducing variation is the 

 simple change of seed, the philosophical reasons for 

 which are explained on earlier pages. A plant becomes 



